A CMP process of chemically mechanically polishing a wafer is performed by rubbing a surface of the wafer against a polishing surface while supplying a polishing liquid (i.e., slurry), containing abrasive grains, onto the polishing surface. The abrasive grains and polishing debris remain on the polished wafer. Thus, the wafer is sent to a cleaning chamber and cleaned in the cleaning chamber. Cleaning of the wafer is performed by supplying of a chemical liquid, rinsing of a wafer with pure water, and the like. The cleaned wafer is further transported to a drying chamber, where the wafer is dried.
The polished wafer may have an exposed silicon on its surface. If the wafer is dried with pure water present on the silicon, a watermark is likely to be formed on the surface of the wafer. A mechanism of the formation of the watermark is generally considered as follows. Oxygen present in the air is dissolved in pure water on a wafer, thus inducing dissolution of silicon in the pure water or inducing dissolution of silicon oxide (e.g., SixOy) in the pure water. When such pure water is vaporized, silicon hydrate remains as a precipitate on the surface of the wafer. This precipitate is the watermark.
The watermark may cause a defect of devices formed on the wafer or a decrease in performance of the devices. However, once the watermark is formed, it is extremely difficult to remove the watermark from the wafer. Therefore, it is very important to prevent the formation of the watermark when drying a wafer.